Many different techniques are available to perform analog-to-digital (A/D) or digital-to-analog (D/A) conversion. Typical converters for performing A/D conversion operate at levels of at least twice the highest frequency component that is being sampled (known as the Nyquist rate). In addition, many ADCs operate at even higher frequencies, known as oversampling converters. The main advantage of the oversampling technique is the fact that the quantization noise, inherently introduced by the A/D conversion, is spread over a wider bandwidth resulting in a lower noise floor in the band of interest. Moreover, if a lower noise floor is not a requirement, then oversampling the A/D conversion can be performed with lower precision, obtaining the same noise floor in the band of interest. For example, by performing oversampling, the ADC output can be a single bit, completely avoiding the need for precisely tuned components. However, oversampling requires that the oversampled information later be reduced to the Nyquist rate.
Oftentimes, an oversampling ADC is formed using a delta-sigma modulator (DSM), which includes at least an integrator, also referred to as a loop filter, and a comparator, also referred to as a quantizer, connected in a negative feedback configuration to provide quantized outputs. The main advantage of delta-sigma modulation is the shift of the quantization noise from the band of interest to another band, a technique known as noise-shaping. A low-pass filter and a decimator may then be used to eliminate the out-of-band noise and provide a digitized signal at the Nyquist rate. Delta-sigma modulators are often designed using a high-order loop filter, as lower sampling rates can be used to obtain the desired precision. However, at high-orders (typically three and above orders), a non-linear response of the DSM that is fed back to the DSM input can cause instability. This instability is a function of the amplitude and frequency of the incoming signal. While it is oftentimes possible to recover from an instability condition by resetting the modulator to a predetermined state, such operation is time consuming and leads to a potential loss of signal information.
Accordingly, a need exists for improved DSM operation.